macos: send stop event when fingers touch

macos: send stop event when fingers touch

When the fingers are placed on the touchpad, we get a scroll event with
the phase NSEventPhaseMayBegin. We can use this to synthesize an is_stop
event. This results in the scrolledwindow stopping scroll with stop
gestures.

This can cause another warning as well, however, which should be addressed
from #4730.

Fixes #4733